Preheat the oven to 400 degrees F.
Cook the pancetta
in a heavy large oven-safe pot over medium heat until crisp
, about 8 minutes. Add the onions
and garlic, and saute until the onions are translucent, about 5 minutes. Mix in the beer
, tomato sauce
, brown sugar
, vinegar, molasses
, mustard, salt, and pepper. Stir in the beans. Bring to a simmer
. Transfer to the oven and bake, uncovered, until the bean mixture bubbles and thickens slightly, stirring occasionally, about 45 minutes.